1. Field of the Invention
The present invention relates to a sunscreen composition comprising an effective amount of zinc oxide and at least one of a carbomer and C10 to C30 alkyl acrylate crosspolymers in a stable emulsion. The present invention also relates to associated processes for producing and using such compositions.
2. Description of Related Art
Exposure to ultraviolet radiation is a major concern as a result of its aging effects and its contribution to the formation of cancerous cells in the skin.
Organic sunscreen agents have been incorporated in a variety of cosmetic applications. As will be appreciated, the primary application of such agents is in tanning and sunscreen lotions. Formulating many personal care products such as tanning and sunscreen lotions is becoming increasingly complex as efforts are made to promote stability and efficacy of the final product. Increasingly sophisticated formulations and processing techniques significantly increase the expense faced by the manufacturer.
Furthermore, there typically exist limits on the concentration and frequency of use of many organic sunscreens. Accordingly, concerns have arisen regarding the potential for irritation resulting from organic sunscreens penetrating into a user""s skin. It is also these limited use levels that have necessitated many formulators to use combinations of different types of sunscreen agents to obtain relatively high sun protection factor (SPF) products.
Physical sunscreens, mainly comprising inorganic pigments, have been utilized to increase SPF and to eliminate or reduce the high concentration levels of organic sunscreens and therefore attempt to reduce the potential for skin irritation. These inorganic pigments are also very effective in providing protecting from UVA radiation.
However, currently known sunscreen formulations comprising physical sunscreen agents or inorganic pigments, present difficult problems for formulators attempting to develop stable emulsion products. Physical sunscreens are typically difficult to maintain in a uniform suspension, in that they readily settle from the liquid carrier and often agglomerate.
One method of increasing emulsion stability has been to incorporate carbomers and C10 to C30 alkyl acrylate crosspolymers into suspension systems. Carbomers and C10 to C30 alkyl acrylate crosspolymers have been used in sunscreens and other cosmetic and personal care products for years as thickeners, stabilizers, and suspending aids.
Titanium dioxide is recognized as a successful physical sunscreen and has been used in a variety of commercially available sunscreen and skin care products. Titanium dioxide has several beneficial characteristics that make it attractive to the formulator and the end user. First, titanium dioxide is relatively inert. It is insoluble in water and dilute acid. As a result. the material will not react with other components in skin care formulations. Second, the properties of titanium dioxide are such that emulsions containing titanium dioxide are relatively stable.
Carbomers and C10 to C30 alkyl acrylate crosspolymers have been successfully used in sunscreen formulations containing titanium dioxide. Compatibility between these polymers and titanium dioxide is due to the fact that titanium dioxide does not dissociate in the aqueous phase. Such compatibility is also due to the fact that titanium dioxide has a negative surface charge which is compatible with the polymers which are generally anionic.
In emulsion-based formulations containing both (i) carbomers and/or C10 to C30 alkyl acrylate crosspolymers and (ii) titanium dioxide, the titanium dioxide may be formulated into either a nonaqueous or an aqueous phase with little concern. The only concern is that the pH of the formulation be kept above the isoelectric point so that agglomeration of the inorganic particles, i.e., titanium dioxide, does not occur.
Although sunscreen compositions based upon titanium dioxide are generally satisfactory, a need exists for an improved physical sunscreen composition, preferably one that utilizes zinc oxide. Specifically, it would be desirable to provide a sunscreen composition able to exhibit relatively high SPF values without the accompanying potential for skin irritation.
Attempts have been made to incorporate zinc oxide, an inorganic or physical sunscreen agent, into various formulations, including sunscreens. Zinc oxide provides significantly greater transparency properties with respect to visible light as compared to titanium dioxide, yet also provides protection from harmful UV radiation. However, as far as is currently known, sunscreen formulations comprising zinc oxide have met only limited success. This is believed to stem from the very reactive nature of zinc oxide and the accompanying problems of formulation instability. Accordingly, it would also be desirable to provide a sunscreen formulation, particularly one utilizing zinc oxide that is relatively stable and so has a long shelf life.
The present invention achieves the foregoing objectives and provides in a first aspect, a sunscreen composition comprising (i) an emulsion vehicle including an aqueous phase and an oil phase, (ii) zinc oxide dispersed in the oil phase, (iii) a crosslinked carboxylic acid polymer, (iv) one or more stabilizing agents incorporated in either or both of the aqueous and oil phases so that the zinc oxide is maintained, or at least substantially so, in the oil phase and substantially precluded from entering the aqueous phase, and (v) an aqueous buffering system with chelating agents to control pH and reduce the migratory tendency of zinc to interact with the carboxylic acid polymer.
In another aspect, the present invention provides a method for significantly improving the stability of a formulation, preferably a sunscreen formulation, that includes an emulsion vehicle, zinc oxide, and a crosslinked carboxylic acid polymer. The method involves maintaining, or at least substantially so, the zinc oxide in the oil phase of the emulsion vehicle.
In yet another aspect, the present invention includes a method for providing protection to an individual""s skin from ultraviolet radiation by applying a composition in accordance with the present invention to a user""s skin that comprises zinc oxide.